Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in ink jet printers are generally classified as either dye-based or pigment-based. In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and settling out of the carrier. The process of preparing inks from pigments commonly involves two steps: (a) a dispersing or milling step to break up the pigment to the desired particle size, and (b) a dilution step in which the dispersed pigment concentrate is diluted with a carrier and other addenda to a working strength ink. In the milling step, the pigment is usually suspended in a carrier (typically the same carrier as that in the finished ink) along with rigid, inert milling media.
Mechanical energy is supplied to this pigment dispersion, and the collisions between the milling media and the pigment cause the pigment particles to be reduced in size. A dispersant or stabilizer, or both, is commonly added to the pigment dispersion to facilitate the deaggregation of the raw pigment, to maintain colloidal particle stability, and to retard particle reagglomeration and settling. Although there are known in the prior art a wide variety of dispersing agents for pigmented ink jet inks, they are not without certain problems. For example, many dispersing agents are very selective as far as being able to disperse pigments to sub-micron size. In many instances each class of pigments may require a specific dispersing agent. Another problem encountered with polymeric dispersing agents is that they tend to impart an undesirable high viscosity to the resulting inks. Thus there is a continuing need for improved dispersing agents for pigmented inks.
U.S. Pat. No. 5,512,968 discloses ammonium and sodium N-methyl-N-oleoyl taurate for use as dispersant in ink jet inks. The problem is that when sodium N-oleoyl-N-methyltaurate (NaOMT) the dispersion foams excessively. This requires use of a defoamer. Ammonium N-oleoyl-N-methyltaurate methyltaurate is not commercially available, and produces NH.sub.3 when the pH of the solution is raised.